In-line high-temperature pH control during hot-water extraction of wood

•An automated pH controlling system was developed for hot-water extractions.•Three different in-line pH set points were tested, 4.3, 4.6 and 4.85.•Hemicellulose yield decreased with higher extraction pH.•Average molar mass of the hemicelluloses increased with higher extraction pH.•Monomeric sugar formation could be inhibited with a extraction pH of 4.85.

An in-line pH control system for hot-water extraction of wood was developed and tested for optimal extraction of high-molar-mass hemicelluloses from spruce wood. Automated alkali addition was achieved by combining a controller and HPLC pump with a solid ZrO2-based high-temperature and high-pressure pH electrode and a high-temperature and high-pressure Ag/AgCl reference electrode. Three different set points were tested; 600 mV, 575 mV and 550 mV corresponding to a pH at 170 °C of 4.3, 4.6, and 4.85, respectively. An extraction with no pH control was conducted for reference. The extracts were analyzed for total dissolved solids, non-cellulosic carbohydrates, monomeric sugars, average weight molar mass of the hemicelluloses and free acetic acid. The results show that higher extraction pH increases the molar mass but decreases the total yield of dissolved hemicelluloses and also inhibit sugar monomer formation.